Optimization of focused spots for maskless lithography

Abstract

The focused spots of an active spot array projection system, such as a maskless lithographic projection system, are optimized within a relay of the projection system. A frequency modulator is positioned proximate to the pupil of the relay for reforming the focused spots while imaging the focused spots onto a photosensitive substrate.

The invention claimed is: 
     
       1. An active spot array projection system comprising
 an illuminator for illuminating addressable elements of a pattern generator, 
 an imager for imaging the addressable elements of the pattern generator onto corresponding focusing elements that form focused spots at focal points of the focusing elements and in a pattern controlled by the pattern generator, 
 a relay for relaying an image of the pattern of focused spots, each containing a range of spatial frequencies, onto a photosensitive substrate, and 
 a frequency modulator proximate to a pupil of the relay for collectively modulating selected spatial frequencies in selected positions appearing inside the pupil of the relay for adjusting a shape of the focused spots on the photosensitive substrate. 
 
     
     
       2. The system of  claim 1  in which the frequency modulator is an apodizer for attenuating certain spatial frequencies of the focused spots. 
     
     
       3. The system of  claim 2  in which the apodizer asymmetrically attenuates certain spatial frequencies about an optical axis of the relay to radially balance spatial frequency distributions within the relay pupil. 
     
     
       4. The system of  claim 2  in which the apodizer asymmetrically attenuates certain spatial frequencies about an optical axis of the relay to reduce asymmetric distributions of spatial frequencies within the relay pupil. 
     
     
       5. The system of  claim 2  in which the apodizer symmetrically attenuates certain spatial frequencies about an optical axis of the relay to reduce side lobes of the focused spots imaged onto the substrate. 
     
     
       6. The system of  claim 1  in which the frequency modulator is a phase plate located proximate to the pupil of the relay for delaying phases of certain spatial frequencies of the focused spots. 
     
     
       7. The system of  claim 6  in which the phase plate asymmetrically delays phases of certain spatial frequencies about an optical axis of the relay to reshape the focused spots imaged onto the substrate. 
     
     
       8. The system of  claim 6  in which the phase plate symmetrically delays phases of certain spatial frequencies about an optical axis of the relay to reduce side lobes of the focused spots imaged onto the substrate or to reduce variations in defocus. 
     
     
       9. The system of  claim 1  in which the frequency modulator is offset from the relay pupil along an optical axis of the relay for disproportionately modulating focused spots in one portion of the pattern of focused spots imaged onto the substrate with respect to another portion of the pattern of focused spots imaged onto the substrate. 
     
     
       10. A method of reforming focused spots within a controllable pattern of focused spots comprising steps of
 illuminating individually addressable elements of a pattern generator with an illuminator, 
 imaging the addressable elements of the pattern generator onto corresponding focusing elements for forming focused spots at focal points of the focusing elements in a pattern controlled by the pattern generator, 
 relaying the pattern of focused spots, each containing a range of spatial frequencies, through a common pupil and onto a substrate image plane, and 
 collectively modulating selected spatial frequencies of the focused spots in selected positions appearing inside the common pupil so that images of the focused spots on the substrate image plane contain altered angular distributions of light for adjusting a shape of the focused spots on the substrate image plane. 
 
     
     
       11. The method of  claim 10  in which the step of collectively modulating includes asymmetrically attenuating certain spatial frequencies about an optical axis of the relay to radially balance spatial frequency distributions within the common pupil. 
     
     
       12. The method of  claim 10  in which the step of collectively modulating includes asymmetrically attenuating certain spatial frequencies about an optical axis of the relay to limit asymmetric distributions of spatial frequencies within the common pupil. 
     
     
       13. The method of  claim 10  in which the step of collectively modulating includes symmetrically attenuating certain spatial frequencies about an optical axis of the relay to reduce side lobes of the focused spots imaged onto the substrate image plane. 
     
     
       14. The method of  claim 10  in which the step of collectively modulating includes asymmetrically delaying phases of certain spatial frequencies about an optical axis of the relay to reshape the focused spots imaged onto the substrate image plane. 
     
     
       15. The method of  claim 10  in which the step of collectively modulating includes symmetrically delaying phases of certain spatial frequencies about an optical axis of the relay to reduce side lobes of the focused spots imaged onto the substrate image plane or to reduce variations in defocus. 
     
     
       16. The method of  claim 10  including a step of offsetting a frequency modulator from the common pupil for disproportionately modulating focused spots in one portion of the pattern of focused spots imaged onto the substrate image plane with respect to another portion of the pattern of focused spots imaged onto the substrate image plane. 
     
     
       17. The method of  claim 10  including a step of determining an orientation at which the imaged spots are elongated at the substrate image plane, and the step of collectively modulating includes modulating spatial frequencies in an orthogonal orientation within the common pupil to reform the imaged spots into a less elongated shape. 
     
     
       18. In a maskless microlithographic projection system that produces a controllable array of focused spots for imaging patterns onto photosensitive substrates, an improvement comprising
 an imager for imaging addressable elements of a pattern generator onto corresponding focusing elements that form an array of focused spots at focal points of the focusing elements and in a pattern controlled by the pattern generator, 
 a frequency modulator positioned within a relay of a projection lens that relays an image of the array of focused spots onto a photosensitive substrate, and 
 the frequency modulator being arranged for collectively modulating selected spatial frequencies of the focused spots in selected positions appearing inside a pupil of the relay. 
 
     
     
       19. The projection system of  claim 18  in which the frequency modulator asymmetrically modulates the selected spatial frequencies about an optical axis of the relay for collectively correcting a shape of the focused spots that are imaged onto the photosensitive substrate. 
     
     
       20. The projection system of  claim 18  in which the frequency modulator symmetrically modulates the selected spatial frequencies about an optical axis of the relay for collectively redistributing light within the focused spots. 
     
     
       21. The projection system of  claim 18  in which the frequency modulator is offset from an aperture stop of the relay for differentially modulating spatial frequencies among the focused spots of the array of focused spots. 
     
     
       22. The projection system of  claim 18  in which the frequency modulator is arranged as an apodizer for asymmetrically attenuating certain spatial frequencies about an optical axis of the relay. 
     
     
       23. The projection system of  claim 18  in which the frequency modulator is arranged as a phase plate for asymmetrically delaying phases of certain spatial frequencies about an optical axis of the relay.